In this article, we demonstrate the properties of a hydrogel capable of adsorbing calcium cations from aqueous streams at ambient temperatures (10–25°C) and showing almost complete desorption of the bound calcium ions at slightly elevated temperatures (40–50°C). Successful breakthrough experiments in a fixed bed column set-up show the potential of the hydrogel in softening water streams from 150 ppm toward <10 ppm hardness levels. The regeneration ability is based on the thermo-reversible internal ion pair–formation mechanism of the hydrogel. Additionally, it was shown that the thermo-reversible adsorption mechanism was successful for magnesium and copper ions too. The exploration of the properties of hydrogel CS-1B14 is the first step in the development of sustainable water-softening processes, both at industrial scale as well as for household applications, using low-grade residual heat to regenerate these gels. In this way, the use of concentrated brine streams, acid/caustic treatment, or high regeneration temperatures is avoided. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40216.